The effect on the webpage stats is very difficult to justify to the Business Intelligence and Marketing guys.

Besides, when I tested this technique, I had to set some cache header on our html. For a social network with much user stories a tricky task.

And finally: isnt this going to kill the backend ? Image Yahoo! prefetches von yahoo.com the mail.yahoo.com page for every firefox user. All users, who will NOT visit mail.yahoo.com generate unnecessary backend traffic. And those would be millions.